Abstract High levels of high density lipoprotein-cholesterol (HDL-C) are associated with lowered risk for cardiovascular disease (CVD) in epidemiological studies. Although several mechanisms may play a role in HDL?s protective effect, HDL and its major protein constituent, apolipoprotein-AI (apoA1), are critical components of the reverse cholesterol transport (RCT) pathway, in which cholesterol is removed from peripheral tissues and transferred to the liver for excretion. In the first step of the RCT pathway, lipid-poor apoA1 acts as an acceptor for cell cholesterol and phospholipids via the cell membrane protein ABCA1, generating nascent HDL. However, not all HDL is equivalent, and several studies have reported that individuals with coronary artery disease have HDL that is ?dysfunctional? and no longer atheroprotective. The atherosclerotic lesion is a highly oxidative environment, and human lesions contain high levels of the antimicrobial enzyme myeloperoxidase (MPO), which we and others have shown can oxidize apoA1 and impair its function. We determined that the four tryptophan residues in human apoA1 (h-apoA1) are crucial in its MPO mediated loss of cholesterol acceptor function. We created a novel 4WF h-apoA1 variant, in which all four tryptophan residues are replaced by phenylalanine, which is resistant to MPO-mediated loss of function. We created and characterized transgenic mice that express high levels of the 4WF h-apoA1 isoform and found that these mice were resistant to inflammation. We also created human MPO transgenic mice, which over express MPO that can be further induced by zymosan treatment. Here we propose to characterize h-apoA1 modifications in mouse models and if these are modulated by inflammatory stimuli. We will also test whether the oxidant resistant 4WF apoA1 isoform can better protect from inflammation and sepsis, promote reverse cholesterol transport, delay atherosclerosis progression, and promote atherosclerosis regression in mice that over express MPO, creating an oxidative environment similar to that found in human lesions. Mechanistically, we will determine how the 4WF isoform protects mice from an acute phase response, and examine if the 4WF isoform better prevents myeloid cell proliferation and mobilization from the bone marrow leading to monocytosis and neutrophilia that are associated with atherosclerosis progression.